The present invention relates generally to geophysical exploration and more particularly to methods for estimating the burial conditions of sedimentary material in the earth's subsurface.
In the continuing search for hydrocarbons in the earth's subsurface formations, explorationists seek to understand the sequence and conditions under which sedimentary materials are buried in order to better locate hydrocarbon deposits as well as to more economically and safely control the drilling process necessary to tap such resources. In particular, geophysicists seek estimates of elements of the burial or compaction conditions of sedimentary materials in the earth's subsurface, including density, lithology, porosity and pore fluid pressure.
While the use by explorationists of the compaction conditions can be extremely useful in evaluating the earth's formations, prior knowledge of in-situ pore fluid pressure can be extremely important to drilling personnel because of the hazards associated with drilling into overpressured formations (i.e., those where pore fluid pressure is greater than normal hydrostatic pressure). The hazard arises because such overpressured fluids can eject themselves up the borehole and out the top, creating a substantial safety hazard to the drilling crew. The Gulf Coast of Mexico is notorious for such overpressured formations and the ability to predict their occurrence can greatly reduce the hazards associated with drilling into such overpressured formations.
Present techniques for estimating in-situ pore fluid pressure at a particular locale typically comprise statistically combining data obtained from a wellbore (e.g., vertical seismic profile (VSP) or sonic or resistivity logging data) and downhole measurements of pore fluid pressures from other locales. Since these methods are empirically derived, it is necessary to determine the coefficients of fit each time the methods are employed at different locales, even within the same sedimentary basin.
The methods for estimating the burial conditions of sedimentary material of the present invention have substantial advantages over existing techniques. The most significant of these advantages is the ability to estimate in-situ pore fluid pressures from surface seismic data alone. Pore fluid pressure predictions obtained in this manner can reduce the danger and added expense of exploratory drilling in overpressured formations. Since the nature of the in-situ pore fluid pressure estimates in the present invention is determinative, rather than empirical, no adjustments of the coefficients of fit are required at different locales within a selected sedimentary basin.